This program is the continuation of a long-term study of the regulation of genes responsible for staphylococcal pathogenesis, collectively known as the virulon. The present application deals with the mechanism of function and interactions of two key signaling systems, agr and sae, on their relation other regulatory elements, and on their relation to clinical disease. It addresses the ways in which these two systems determine the specificities of in vitro and in vivo behavior - the lifestyles of the organism, Specific Aims are 1. To determine the mechanism by which agr-RNAIII regulates target gene expression. 2. To analyze the regulatory functions of the saeRS system and its interactions with agr. 3. To determine the in vivo functionality and interactions of these two regulatory systems. In Aim 1, a variety of in vivo and in vitro techniques will be used to identify the intermediary transcription factors that are directly regulated by RNAIII, focusing on regulation of translation. In Aim 2, gene fusions and deletions will be used to characterize the regulatory organization of the sae locus and its interaction with agr, and with other regulatory inputs. In Aim 3, the function of selected regulatory genes in vitro will be compared with activities in vivo. Accomplishment of these objectives will not only enhance our basic scientific understanding of the interaction between a pathogenic organism and its host, but may also contribute to our ability to control the organism so as to treat or prevent infections.